Amplifiers are widely used to amplify electric signals received from one part of a circuit for use in another part of the circuit. Commonly, the electrical signal to be amplified is an alternating current (AC) signal exhibiting a time-varying electric potential or voltage. The amplifier is then required to preserve the time varying nature of the voltage, while simultaneously amplifying the voltage by some controlled factor or gain.
As suggested, amplifiers are used in a variety of applications. For example, amplifiers are commonly employed in communication systems to amplify signals received from an antenna for processing by a receiver, or to amplify signals received from a transmitter for broadcast via the antenna. Similarly, amplifiers are used in radar systems to amplify signals received from, and applied to, the system antennas.
Because the construction and operation of amplifiers also varies considerably, amplifiers are commonly separated into classes on the basis of their performance characteristics. For example, Class A amplifiers are biased for maximum power output even when substantially no AC input is received and, as a result, have a relatively low efficiency on the order of 25 percent. In contrast, Class B amplifiers are advantageously biased near cutoff when no AC input signal is applied and have a higher efficiency on the order of 60 percent.
For the purposes of the ensuing discussion, one class of amplifier that is of particular interest is the Class E amplifier. Class E amplifiers are commonly used in switching power supplies and are designed to deliver maximum power to the load, rather than maximum voltage gain. Such amplifiers are, therefore, referred to as Class E "power" amplifiers.
The general construction and operation of Class E power amplifiers for radio-frequency (RF) applications is discussed in, for example, Avratoglou et al., Analysis and Design of a Generalized Class E Tuned Power Amplifier, IEEE Transactions on Circuits and Systems, Vol. 36, pp. 1068-79 (1989) and Kazimierczuk et al., Class E Tuned Power Amplifier with Antiparallel Diode or Series Diode at Switch, with Any Loaded Q and Switch Duty Cycle, IEEE Transactions on Circuits and Systems, Vol. 36, pp. 1201-09 (1989).
In that regard, a conventional Class E amplifier may include a transistor switch, load network, and radio-frequency (RF) choke. The load network is represented by a resistive load R, a switch parasitic capacitance and parasitic shunt capacitance C.sub.1, and a series inductance L and capacitance C. The RF choke is included to ensure a substantially direct current (DC) supply.
The amplifier operates in the following manner. When the switch is closed, a series resonant circuit including the inductance L, capacitance C, and load R is formed and an output current flows through the switch. When the switch is open, a series resonant circuit including the inductance L, capacitance C, load R, and capacitance C.sub.1 is formed and an output current flows through the capacitance C.sub.1, producing a voltage V.sub.S across the switch. For efficient operation, the voltage V.sub.S and its rate of change are preferably zero when the switch closes, ensuring that no energy will be stored in capacitance C.sub.1. The efficiency of such amplifiers may be on the order of roughly 90 percent.
As will be appreciated, the efficient operation of the Class E amplifier is advantageous for a number of reasons. For example, an amplifier having a relatively high efficiency dissipates less heat than an amplifier of lower efficiency, reducing circuit cooling requirements and typically extending amplifier life. In addition, a relatively efficient amplifier allows higher RF power output levels to be achieved for a given input power.
While higher output levels can sometimes be achieved by increasing the input current, such adjustments are not always practical. For example, an aircraft communication or radar system is typically wired for some preset maximum current level that might be exceeded by an increase in input current levels. By using a more efficient amplifier, however, higher power output levels could be achieved without requiring the system to be rewired.
Unfortunately, conventional Class E amplifiers cannot necessarily be used to full advantage in many communication and radar applications. For example, a Class E amplifier operates efficiently when its output impedance is either very low (near zero ohms) or very high (an open circuit). As a practical matter, however, the output impedance commonly varies with time and frequency. Because the operating frequency of many communication and radar systems may intentionally be adjusted over broad ranges, the operation of a conventional Class E amplifier in such systems will be adversely affected.
In view of the foregoing discussion, it would be desirable to provide an amplifier exhibiting a relatively high efficiency. It would further be desirable for the efficiency of the amplifier to be substantially independent of variations in, for example, the operating frequency of the amplifier.